Antenna circuit



Dec. 1, 1936. s HARMON 2,062,414

ANTENNA CIRCUIT Original Filed April 13, 1952 6H0: W214 i Patented Dec. 1, 1936 ANTENNA CIRCUIT Walter S. Harmon, Chicago, Ill., assignor, by

mesne assignments, to Radio Corporation of America, New York, Delaware N. Y., a corporation of Application April 13, 1932, Serial No. 604,896 Renewed October 18, 1935 13 Claims.

This invention relates to an antenna circuit for radio receivers, and more particularly to an antenna circuit having flexibility of design and capable of desirable gain characteristics.

An object of this invention is to provide a coupling circuit for the antenna stage of a radio receiver whose gain curve may easily be made complementary to the gain curves of the succeeding stages so as to produce a flat resultant gain curve for the receiver, or a resultant gain curve having somewhat more gain at the low frequencies than at the high frequencies. This has been accomplished by providing a primary circuit for the antenna stage having an auxiliary coil therein with a predetermined resonant frequency.

Another object of this invention is to provide an antenna circuit having characteristics such that different antenna capacities do not appreciably affect or change the capacity reflected into the secondary, and hence the tracking of the antenna stage with the succeeding stages of the receiver. This has been accomplished by providing a primary circuit having a resonant frequency near or below the low frequency end of the band so that a small change in capacity across the primary circuit has very little effect upon the secondary resonant circuit.

Another object of this invention is to provide an antenna circuit which, while providing the features expressed above will also give a substantial gain at all points of the band. This has been accomplished through the use of an auxiliary coil in the primary circuit not necessarily coupled with the secondary coil, and while lowering the resonant frequency of the total primary circuit does not affect the gain ratio between the primary and the secondary coils.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein a preferred embodiment of one form of the present invention is clearly shown.

In the drawing:

Fig. 1 represents the preferred form of the present invention.

,Fig. 2 represents a modification.

Figs. 3 and 4 represent examples of resonance curves for the coils of the primary circuit and. for the total primary circuit.

Fig. 5 represents examples of a gain curve and variations thereof.

With particular reference to the drawing, and Fig. 1 in particular, an inductance Ill and a primary I4 of a coupling unit iii are connected in series and rare in series with an antenna l and a ground 2. A capacity I2 is connected across the inductance l0, "and may be either an added capacity, or the distributed capacity of theinductance. A capacity 3 is connected between the antenna from a terminal 4 and the ground from a terminal 5. Capacity 3 may be either a capacity of the antenna or the capacity of the antenna and an added capacity. .The primary I4 of the coupling device 16 is magnetically coupled with a secondary inductance I 8 of. the same device. The secondary I8 is also grounded at 2 through a terminal 6 and in the present modification is tunable by a variable condenser 20, butmight be tunable by any other. suitable means and stillcome within the scope of the present invention.

In Fig. 2, inductance III is connected in series with a resonant circuit 24 composed of an inductance 22 and a variable capacity 20, to form a circuit connected at one end to the antenna. l and at the other end to a ground 2. Inductance ID, in this'case also, has a capacity I2 connected thereacross, and may be either the distributed capacity of the inductance or an added capacity. Capacity 3 connected between the antenna from a terminal land the ground from a terminal 5 may be either the antennacapacity or an added capacity or both. An impedance 26 having a variable element 28 is connected between the antenna from terminal land ground from terminal 5.

In Fig. 3 curves A and B represent resonance curves of coils l4 and ll] of Fig. 1 respectively, applied with frequency as the abscissa and voltage as the ordinate. Curve C then represents the resonance curve of the two coils l0 and I4 in series and tuned by the capacity 3. Z may be taken as the upper end of the frequencyband to be received and Y and X as frequencies higher than the upper limit of the band. U, in like manner, may represent the lower limit of the'frev quency band to be received. Similarly, in Fig. 4 curves AA and BB may represent resonance curves of coils l4 and I0 respectively for a different design of said coils or with a difierentcapacity l2 used with coil l0. Curve CC will then represent the resonance curve of the two coils connected in series and perhaps with a different capacity 3 connected thereacross. In this figure 2 represents the upper limit of the band to be received and YY and XX frequencies above that limit. U represents the lower limit of the band and W a frequency below that limit. In Fig. '5

the curves G and GG are plottedwithfrequency:

as the abscissa and gain as the ordinate, where gain is the ratio of E2 to E1.

It is apparent to one skilled in the art that the gain curve of the antenna stage of a radio receiver should be so related to the gain curves of the following stages that the over-all gain of the receiver is practically fiat or if sloping, preferably greater at the low frequency end of the frequency band. In the present invention the inductance I and its capacity l2 may have a resonance curve as shown by B. In such a case it is apparent that the higher the frequency of a signal received, the greater the voltage drop across inductance l0. Hence, as the frequency increases the voltage drop across the primary M will be reduced. As is indicated by comparison of the curves of Fig. 3 and Fig. 4, a change in the resonant frequency in coil II] from a frequency X to frequency XX, nearer the high frequency limit of the band,wil1 increase the voltage drop across inductance l0 and consequently reduce the drop across the primary l4.

The resonant frequency of inductance 10 might be varied through the design of the coil by changing its distributed capacity, or by changing the value of a condenser used to tune that inductance. The resonant frequency of the primary l4 might also be varied to change the turn ratio between it and the secondary 18. By keeping the resonant frequency of the two coils I0 and M in series, low as indicated by curve C or CC, a change in capacity 3 will have negligible effect upon the resonant secondary circuit. Hence, different antenna capacities will not appreciably affect the tracking of the tunable circuit of the antenna stage with those of the other stages.

Since the inductance I0 is not a continuation of the primary 4, and is not directly magnetically coupled with the secondary I8, changes in the design or resonant frequency of the inductance l0 do not change the gain ratio between the primary [4 and secondary 18. For that reason, the coupling unit l6 may be designed to give a substantial gain, and the inductance I!) designed-to determine the general slope of the gain curve. It is apparent from the resonance curves A and AA that if the inductance I 0 were not in the primary circuit the voltage across the primary i4 would increase as the frequency increased, thereby giving a gain curve having an increased gain at the high frequency. It is also apparent that if the gain curve slope were changed by changing the design of the primary I4 only, the ratio between the primary [4 and secondary l8 would have to be changed. If the inductance II] were not used, and the resonant frequency of the primary circuit made low enough so that different antenna capacity would not appreciably affect the tracking of the secondary circuit, the gain ratio would necessarily be greatly reduced.

In the modification shown in Fig. 2, the antenna is directly coupled to the tunable resonant circuit through the inductance I 0. In this modification also, the slope of the voltage transfer ratio E2 to E1 is affected by the design of the inductance ID. The impedance 26 having a variable element 28 connected across the circuit from terminal 4 to terminal forms a variable by-pass circuit to by-pass part of the signal energy an regulate the voltage transfer.

While the form of embodiment of the present invention as herein disclosed; constitutes a preferred form, it is to be understood that other forms might be adopted, all coming within the scope of the claims which follow,

What is claimed is as follows:

1. An antenna circuit for a radio receiver adapted to receive signals within a predetermined frequency band including in combination, a primary circuit having a plurality of inductances, effective capacity across one of said inductances to tune said inductance to a frequency above, but not greatly above, the frequency band to be received, effective capacity for tuning the plurality of inductances to a frequency below the frequency band to be received, a secondary circuit coupled with said primary circuit and tuned by a variable capacity.

2. An antenna circuit for a radio receiver adapted to receive signals within a predetermined frequency band including in combination, an

antenna, a ground connection, a primary circuit connected in series with the antenna and ground, said primary circuit having a plurality of inductances in series, effective capacity across one of said inductances for tuning said inductance to a frequency which is slightly above the frequency band to be received, effective capacity for tuning the primary circuit as a whole to a frequency which is substantially at the lower end of the frequency band to be received and a tunable secondary circuit coupled with the primary circuit.

3. An antenna circuit for a radio receiver adapted to receive signals within a predetermined frequency band including in combination, an antenna, a ground, a plurality of inductances in series with the antenna and ground, means including a fixed condenser connected across one of said inductances for tuning the inductances to a frequency above the frequency band to be received, means including a variable condenser shunting another of the inductances for tuning said other inductance to any desired frequency within the band of frequencies to be received, a capacity shunted across said series inductances for tuning said series inductances as a whole to a frequency which is below the frequency band to be received and means connected across the plurality of inductances to form a continuously variable by-pass circuit.

4. An input circuit for a radio receiver adapted to receive signals within a predetermined frequency band comprising in combination a primary circuit having at least two inductances, effective capacity for tuning each of said inductances to a frequency above the frequency band to be received, at least one of said inductances being tuned to a frequency which is above but close to the highest frequency of said band, effective capacity for tuning the two inductances together to a frequency which is substantially at the lower end of the frequency band to be received, a secondary circuit coupled to the primary circuit and means comprising a variable condenser for tuning the secondary circuit to substantially the frequency of the desired signals.

5. An antenna circuit for radio receivers and the like adapted to receive signals within a predetermined frequency band comprising in combination, a primary circuit having at least two inductances in series but uncoupled with respect to each other, capacity including the distributed capacity of the respective inductances for individually tuning each inductance to a frequency which is above the frequency band to be received, at least one of said inductances being tuned to a frequency which is slightly above the highest frequency of said band, effective capacity including the antenna capacity for tuning the two inductances in series to a frequency which is in the vicinity of the lower end of the frequency band to be received, a secondary circuit coupled to said primary circuit and means for tuning said secondary circuit.

6. The combination with a radio receiving system of an input circuit tunable to select desired frequencies throughout a broad tuning range, a collector of signal energy, a coupling circuit of substantially fixed nature connected between the collector and the input circuit, said coupling circuit being provided with reactance means for such a value that the coupling circuit in combination with said collector is given accentuated response characteristics to Signal energy at a point which is in the vicinity of the lower end of the tuning range and at a point which is slightly above the upper end of the tuning range.

'7. The system described in the next preceding claim further characterized by that the reactance means of the coupling circuit comprises at least two inductance coils one of which coils is used to couple the coupling circuit to the input circuit.

8. The system described in claim 6 further.

characterized by that the coupling circuit comprises at least two inductance coils in series, one of said coils being utilized to couple the coupling circuit to the input circuit and effective capacity across the last named coil for tuning the coil to a frequency which is slightly above the tuning range.

9. The system described in claim 6 further characterized by that the coupling circuit comprises at least two inductance coils in series arranged so as to be connected between the original collector and ground, one of the coils being utilized to transfer energy from the coupling circuit to the input circuit, effective capacity across the last named coil to tune said coil to a frequency which is slightly above the highest frequency of said tuning range and effective capacity across the other of said inductance coils for tuning said other inductance coil to a frequency which is higher than the frequency to which the first inductance coil is tuned.

10. Means for providing a substantially constant transfer of energy between a source of energy and a utilizing circuit throughout a broad tuning range comprising means for passing the energy through a path, a portion of which comprises means for tuning said portion of the path so as to be resonant at a frequency which is slightly above the highest frequency of said range and additional means in said path for tuningthe path as a whole to a frequency which is in the vicinity of the lowest frequency of the range.

11. The arrangement described in the next preceding claim further characterized by that a signal selector circuit is provided together with means for feeding energy from the path to the selector circuit whereby the energy after being passed through the path is resonated in the signal selector circuit.

12. The combination with a radio receiving system of an input circuit tunable to select desired frequencies throughout a broad tuning range, a collector of signal energy, a coupling circuit of substantially fixed nature connected between the collector and the input circuit, said coupling circuit being provided with tuning reactances of such a value that the coupling circuit in combination with the collector is given a response characteristic to signal energy which is accentuated at a frequency which is in thevicinity of the lowest frequency of the tuning range and at a frequency which is slightly above the highest frequency of the tuning range.

13. The system described in the next preceding claim further characterized by that the antenna is connected to ground through a variable resistance device whereby any predetermined amount of the available signal energy may be by-passed to ground.

WALTER S. HARMON. 

